1. Field of the Invention
This invention relates to solar heating systems and, more particularly, to a solar heating system which delivers heated distribution fluid directly to a heat-pump heat exchanger or a ground conduit without causing excessive drying of the ground surrounding the conduit.
2. Description of the Prior Art
Heating systems utilizing solar heat collected by solar panels have long been in common use. In these conventional systems, a fluid flows through the solar panel, which has a relatively large surface area to maximize the quantity of solar heat absorbed by the distribution fluid. The heated distribution fluid then flows to a heat-storage device, which is typically a large insulated reservoir of the distribution fluid or a large quantity of heat-storage material, such as rocks, through which the distribution fluid flows. Heat is then transferred to a heat-utilization device, either directly, by the distribution fluid, or indirectly, through a heat exchanger. These conventional solar heating systems generally utilize a differential thermostat which causes the distribution fluid to flow only when the temperature of the fluid leaving the solar panel is significantly greater than the temperature of the fluid in the heat-storage device.
Attempts have been made to supplement the efficiency of solar heating systems by utilizing the ground as a source of heat and as a means of storing heat generated by the solar panels. Accordingly, a length of conduit is buried in the ground and the distribution fluid recirculates through the conduit and the heat-storage device. The heat-storage device thus acts as a manifold, receiving distribution fluid from and discharging distribution fluid to the solar panels, ground conduit and heat-utilization device. Although such enhancements have, in fact, improved the efficiency of solar heating systems, it has not been heretofore recognized that the efficiency of such systems has not yet been optimized. The quantity of heat transferred from the ground to the ground conduit is a function of the temperature of the ground, the "thermal mass" or specific heat of the ground, and the thermal conductivity of the ground. The ground is typically quite moist so that its thermal mass and thermal conductivity are fairly high. Thus, as the temperature of the ground is raised, a substantial quantity of heat is stored. High ground moisture also causes the thermal conductivity to be fairly high so that a relatively small temperature difference between soil and ground conduit is required to transfer a given amount of heat, and the effective heat-drawing radius of the conduit is relatively high. However, as the temperature is raised further, beyond a predetermined value which varies from area-to-area, the increased temperature removes moisture from the ground. Thereafter, although the temperature of the ground continues to increase, its thermal mass markedly decreases so that the quantity of heat stored in the ground actually decreases with an increase in temperature. More importantly, however, the reduced thermal conductivity effectively insulates the conduit from the ground. Thereafter, a greater temperature differential between conduit and ground is required to transfer a given amount of heat, and heat is absorbed from a much smaller area. Conventional solar heating systems utilizing ground conduit enhancement have not heretofore operated in a manner which prevents detrimental drying of the ground. As a result, the efficiency of these heating systems is far from optimum.